1. Field of the Invention
The present invention relates generally to a medical implant, and specifically to a device for detecting a magnetic field in the vicinity of such an implant.
2. Description of the Prior Art
In a medical implant, such as a pacemaker, a magnetic field detector is used for non-invasive activation of different functions in the implant in combination with a permanent magnet placed in the vicinity of the implant at the outside of the patient's body. Some of the functions which can be activated in, e.g., a pacemaker are: disabling the pacemaker's demand function so the pacemaker adapts its operation to battery capacity and having the pacemaker operate in a special, temporary stimulation mode, e.g., in the case of tachycardia, and in conjunction with pacemaker programming.
Outside the implant art, the detection of magnetic fields in a number of different ways, e.g., with the aid of reed switches, by changing the resonance frequency or inductance, etc., is generally known.
One device for determining the strength of a magnetic field is described in an article by Lennart Grahm, "Elektrisk matteknik, Analoga instrument och matmetoder," part 2, 1977, Elektrisk matteknik, Lund, pp. 543-545. As described therein, the voltage induced in a small test body made of ferromagnetic metal is examined with a Forster probe. The Forster probe consists of a small test body made of a ferromagnetic material with high permeability and provided with two windings, one of which is used for alternating current magnetization and the other is used for measuring the ensuing induced voltage. The larger the constant magnetic field, the greater the amplitude of even harmonics when the probe is placed in a constant magnetic field. Thus, a phase detector with a reference voltage equal to twice the frequency of the excitation current can be used for supplying a signal which increases with an increase in the constant magnetic field.
In the implant art, a conventional magnetic field detector consists of a reed switch. Reed switches, however, are sensitive and rather expensive components which also occupy a relatively large amount of space in the implant.
In order to eliminate the need for a reed switch, therefore, recent proposals have suggested utilization of the implant's telemetry unit so that the unit can also be used for detecting the presence of a magnetic field, in addition to its telemetry function.
U.S. Pat. No. 4,541,431 discloses one such proposal with a combined telemetry and magnetic field detector unit. This unit contains a conventional resonant circuit containing, e.g., a coil used in telemetry for transmitting and receiving data. The resonant circuit is also used for sensing the presence of a magnetic field whose strength exceeds a predefined value. The resonant frequency for the resonant circuit varies with the strength of the magnetic field. The resonant circuit is periodically activated, and the number of zero crossings of its signal with a sensing window with a predefined duration is determined. If a predetermined number of zero crossings occurs, this means that the strength of the magnetic field exceeds the predefined value.
Thus, the known unit requires a number of components for detecting the frequency deviation, and thus the magnetic field.